Add server 2012 R2 Essentials to existing SBS 2003 domain

Is it possible to have SBS 2003 in its role as exchange server, may be on another subnet,
coexisting with Server 2012 Essentials?

Ok. Just to be sure: Server 2012 R2 Standard will join SBS2003 as a member without problems?

It is worth pointing out that while Windows Server 2003 is not EOL for 2.5 months, Exchange 2003 (a component in SBS) has been EOL for over a year.  That means any security vulnerabilities in Exchange 2003 (including the SMTP engine, OWA, Exchange ActiveSync) have *not* been patched and are considered vulnerable.  For this reason, SBS is effectively already EOL and is already insecure, even if the underlying OS still has a small breath of life left. Any server running Exchange 2003, including SBS 2003, should've been retired a year ago.

Do not worry about expiration. If everything works fine, there is no problem. The point is, that Microsoft no longer combines Exchange with server software. So the smooth migration is to first migrate what IS included, and keep what is not included. I asume that Lee W. MVP must be correct, that there is a possibillity for coexistense of Server 2012 essentials with SBS 2003 in the same network, with different subnets and domains.  However, I fear it will be complicated to maintain. So may be the best implementation is to use the Standard version of 2012 R2, and make it a member for the time being. 21 days is too short (which is the grace period for essentials).

Splitting the servers into different domains and subnets doesn't address the legal aspect of the essentials and SBS licensing restrictions so I don't bring them up in maintaining EE's policy regarding discussions of sidestepping technical licensing limitations.

As always, licensing advice is not legally binding, but my (fairly confident) interpretation of the SBS and essentials licenses is that even if you take steps to separate them via subnets, etc, you'd still be violating their use clause by having both. Thus illegal.

It is not easy to find the licensing agreements on the internet. However, I think it is not possible to pose such restrictions to the owner of a license.  If someones has bought some version of software, buying another version  of some other software CANNOT make the older version illegal without buying it back. So "having both" cannot be illegal. The only restriction which can legally be made in a licenseagreement is in how you can use it in combination.
Such type of restriction is present for Terminal server (it must run on a separate server; technically that is not a requirement, because in evaluation it runs on the domaincontroller without problems).
I just respond on this statement, because I hate illegal licensing restrictions, I assume, Microsoft is a legal company.

I have started installing server 2012 R2 essentials, and read the license agreement. Luckily there is no restriction whatsoever in how many of them you have and use, may be in one company.
 The restriction is: It is the only domain controller in the network, and has no trustrelation with another domain (and of course: one instance, 25 users).  The exclusion of a trustrelationship with another domaincontroller makes it unattractive to have two of these for one group of users. Every client must access the controllers individually then.
My original question was raised because of the choice I get, when installing Server 2012 R2 Essentials: Step 2: Configure server as domaincontroller. If you want to join as a member of another domain, read... (a deployment guide).
The deploymentguide then states, that you can join 2012 servers only, and you can join domains temporarily to migrate.
With the information of EE now present, the balance can be made: With Essentials, you loose Exchange. So, even in case of a small company with 15 users, you should use server 2012 Standard to be able to have Exchange.